Because inner and outer wheels rotate at different speeds when turning a vehicle, torsion in the driving system negatively affects driving performance. Accordingly, a differential gear is used in the conventional powertrain. However, if one drive wheel is on a slippery surface, such as ice, snow, or mud, a coefficient of road surface friction decreases on the wheel and it can easily slip even with a small amount of torque. To prevent this problem, a limited slip differential is used.
A power lock-type limited slip differential is most commonly used. This type of differential uses a plurality of friction clutches to perform its slip-preventing function. As a result, the power lock-type limited slip differential has a complicated structure and is heavy.
In a 4-wheel drive vehicle, a specific tight corner braking phenomenon occurs due to a difference of radius of gyration between the front and rear wheels, and more particularly due to the difference of the number of rotations according to the big radius of gyration for the front wheel and the small radius of gyration for the rear wheel when turning the vehicle.
The difference of radius of gyration causes forced slipping of the wheels or torsion stress of the driving system in parts like the propeller shaft. This results in locking the driving system and thereby braking the vehicle. The braking of the driving system has a harmful influence upon rigidity of the propeller shaft and also on smooth turning of the vehicle, and causes loss of power.
As the rear wheels of the 4-wheel drive vehicle have small rotative speed slip because of the speed difference between the front and rear wheels when turning the vehicle, driving performance and stability of the vehicle deteriorate.
In addition, because torsion stress occurs while turning the vehicle causing the vehicle body to vibrate, when designing the vehicle, the size and weight of the propeller shaft should be increased to reduce vibration.
A center differential gear has been used to solve this problem. However, a lock system must additionally be mounted to temporarily stop the differential operation.
In recent years, a viscous coupling has been used to solve the problem. However the viscous coupling has a complex structure and increases manufacturing costs.